Conductive silicone rubber compositions

Abstract: A compound having a group -N=N-, such as a benzotriazole, is added to a conductive silicone rubber composition comprising an organopolysiloxane, conductive carbon black, and an organic peroxide. The composition permits of atmospheric hot air vulcanisation with the aid of an organic peroxide, irrespective of the inclusion of carbon black, and has good shelf stability.

Description: This invention relates to a conductive silicone rubber composition containing conductive carbon black, and more preferably to conductive silicone rubber compositions which have good storage stability and cure into a satisfactory rubber product through atmospheric hot air vulcanization (HAV). Most rubbery materials are electric insulators. A number of conductive rubbers are available as mixtures of rubbery material and conductive agents. For example, rubbers having carbon black blended therein to provide an electric resistivity of from 10 5 to 10 Ω-cm are conductive rubbers which are used in various applications. Silicone rubber is also widely used as electrically insulating rubber because of its high-temperature resistance, low-temperature resistance and weatherability. It can also be used as conductive silicone rubber by adding conductive agents, as with other rubbery materials. The conductive agents which are added to silicone rubber for imparting electric conductivity are typically carbon black, graphite, various metal powders such as silver, nickel, and copper, various non-conductive particles and short fibers surface treated with silver or similar metals, carbon fibers, and metallic fibers. By mixing these conductive agents, the resistivity of silicone rubber can be reduced to the order of 10 10 to 10 -3 Ω-cm depending on the type and amount of conductive agent without detracting from the inherent properties of silicone rubber. Among others, carbon black and metal powders such as silver and nickel are often used where it is desired to produce highly conductive silicone rubber having a resistivity of less than 10 5 Ω-cm. Carbon black is most often used because of cost. However, only a limited vulcanization system is applicable to conductive silicone rubber compositions having blended therein acetylene black and other carbon black as a conductive agent when they are continuously molded as by extrusion molding and vulcanized into a length of article such as a seal, gasket and roll. More particularly, typical organic peroxide vulcanization fails to yield satisfactory molded products. If acyl series peroxides commonly used in atmospheric hot air vulcanization (HAV) of conventional silicone rubber compositions, for example, benzoyl peroxide and 2,4-dichlorobenzoyl peroxide are used in carbon black-containing systems, the carbon black acts to restrain vulcanization. If alkyl series peroxides, for example, dicumyl peroxide and di-tert-butyl peroxide are used in carbon black-containing systems, compression molding is permissible, but extrusion atmospheric hot air vulcanization fails to yield satisfactory products because the surface is vulcanized short due to the influence of oxygen in air. For extrusion atmospheric hot air vulcanization of carbon black-containing silicone rubber compositions, an addition vulcanization approach was conventionally used in order to overcome the above-mentioned difficulty. This approach is to add a platinum series addition reaction catalyst to a mixture of an organopolysiloxane having an alkenyl group and an organohydrogenpolysiloxane having a silicon-attached hydrogen atom capable of addition reaction with the alkenyl group, thus inducing addition reaction curing. This addition reaction curing approach, however, suffers from the problems of a limited shelf life and poisoning by catalyst poisons such as amines, sulfur and tin. This imposes a serious problem in molding. Typically conductive silicone rubber is used to mold a variety of articles, for example, electromagnetic radiation shielding gaskets, building gaskets, business machine conductive rolls (including charging rolls, transfer rolls, developing rolls, paper feed rolls and fixing rolls), and conductive calendered moldings (such as molded zebra connectors). If conductive silicone rubber is molded by conventional atmospheric hot air vulcanization (HAV), problems arise with respect to moldability and rubber quality. It was thus desired to overcome the above-mentioned problems. EP-A-493112 discloses conductive silicone rubber compositions comprising (A) an organopolysiloxane of the general formula (1): R 1 / aSiO (4-a)/2 wherein R 1 groups which may be identical or different are selected from substituted or unsubstituted monovalent hydrocarbon groups and letter a is a positive number of 1.90 to 2.05, (B) conductive carbon black, and (C) an organic peroxide of the general formula (2): wherein X is a group of formula (4) or (5): -(CH 2 ) m - wherein m is an integer of 2 to 8, and R 2 and R 3 are independently a monovalent hydrocarbon group having 3 to 10 carbon atoms or a group -SiR 4 / 3 wherein R 4 is a methyl, ethyl or phenyl group, and (D) an azo-compound as foaming agent. This conductive silicone rubber composition can be extrusion or calender molded and satisfactorily vulcanized by atmospheric hot air vulcanization, that is, effectively molded and vulcanized into silicone rubber having good physical properties and conductivity. Continuing research work, we found that the previously proposed conductive silicone rubber composition exhibited satisfactory physical properties and workability immediately after its preparation, but experienced an increase of plasticity and scorching phenomenon with the lapse of time. There is a need for increasing the shelf stability of such a composition. A general aim herein is to provide new and useful conductive silicone rubber compositions and methods for their preparation. A particular preferred aim is to provide means for giving storage stability to such a composition. The relevant preferred type of composition can be effectively molded and vulcanised into conductive silicone rubber having satisfactory physical properties by extrusion molding or calendering and HAV techniques. We found that vulcanisable, organic peroxide-containing conductive silicone rubber compositions such as in EP-A-493112 could be improved by blending with a triazole or triazine. The resulting compositions remained stable over a long period of storage while maintaining their existing advantages. We also found that another variant of the organic peroxide of formula (2) is useful in giving a vulcanisable composition. According to the invention as set out in claim 1 there is provided a conductive silicone rubber composition comprising: (A) 100 parts by weight organopolysiloxane of the general formula (1): R 1 / aSiO (4-a)/2 wherein groups R 1 , which may be identical or different, are selected from substituted and unsubstituted monovalent hydrocarbon groups and a is a positive number in the range 1.90 to 2.05; (B) 5 to 100 parts conductive carbon black, (C) 0.1 to 5 parts of at least one organic peroxide selected from those according to the general formulae (2) and (3): wherein X is a group of formula (4) or (5): - (CH 2 ) m - m being an integer of 2 to 8, R 2 and R 3 are independently selected from monovalent hydrocarbon groups having from 3 to 10 carbon atoms and groups -SiR 4 / 3 wherein R 4 is methyl, ethyl or phenyl, and n is an integer of 1 to 4, and (D) 0.01 to 5 parts by weight, per 100 parts by weight of compounds (A), (B) and (C) combined, of at least one triazole or triazine compound. Conductive silicone rubber compositions comprising organopolysiloxane of formula (1), carbon black, organic peroxide of formula (2) or (3), and triazine or triazole compound having a group -N=N-, could we found be extrusion molded or calendered, and effectively high frequency (UHF) vulcanisation irrespective of the inclusion of conductive carbon black because the organic peroxide was not adversely affected by carbon black. The compositions, which do not rely on addition vulcanisation, eliminate any concern about catalyst poisons and are thus easy to handle. They are found to be effectively moldable, experience little change of plasticity and little or no scorching phenomenon with the lapse of time after preparation, and maintain stable workability over a long period of time. The compositions are curable to silicone rubber which has a good conductivity and physical properties and is surface tack-free. Component (A) of the conductive silicone rubber composition is organopolysiloxane of the general formula (1): R 1 / aSiO (4-a)/2 R 1 , which may be uniform or varying through the molecule, is selected from substituted or unsubstituted monovalent hydrocarbon groups preferably having 1 to 10 carbon atoms and letter a is a positive number of 1.90 to 2.05. Examples of the monovalent hydrocarbon group include alkyl groups such as methyl, ethyl, propyl and butyl groups; alkenyl groups such as vinyl, allyl and butenyl groups; aryl groups such as phenyl and tolyl groups; and substituted ones of these groups wherein some or all of the hydrogen atoms attached to carbon atoms are replaced by non-interfering substituents such as halogen atoms, cyano groups or the like; such as chloromethyl, chloropropyl, 3,3,3-trifluoropropyl and 2-cyanoethyl groups. The organopolysiloxane(s) should preferably have a linear molecular structure although a partially branched structure is acceptable. The organopolysiloxanes are often blocked with a triorganosilyl or hydroxyl group at the end of their molecular chain. Examples of the triorganosilyl group include trimethylsilyl, dimethylvinylsilyl, methylphenylvinylsilyl, methyldiphenylsilyl, methyldivinylsilyl, and trivinylsilyl groups. For further reducing surface tack, organopolysiloxanes blocked with a polyfunctional group such as (CH 2 =CH) 2 RSi- and (CH 2 =CH) 3 Si- wherein R has the same meaning as R 1 at either end of their molecular chain are preferred. The organopolysiloxanes should preferably have a viscosity of at least 300 centistokes (cs), preferably 100,000 cs or more at 25°C although the degree of polymerization is not critical. Component (B) is electroconductive carbon black which may be selected from various types of conductive carbon black commonly used in conductive rubber compositions. Examples include acetylene black, conducting furnace black (CF), super conducting furnace black (SCF), extra conducting furnace black (XCF), conducting channel black (CC), and furnace black and channel black heat treated at elevated temperatures of about 1500°C. More particularly, the acetylene black include Denka® Acetylene Black manufactured by Denki Kagaku K.K., Shawinigan® Acetylene Black manufactured by Shawinigan Chemical Co., the conducting furnace black includes Continex® CF manufactured by Continental Carbon Co., and Vulcan® C manufactured by Cabot Corp., the super conducting furnace black includes Continex® SCF manufactured by Continental Carbon Co. and Vulcan® SC manufactured by Cabot Corp., the extra conducting furnace black includes Asahi® HS-500 manufactured by Asahi Carbon K.K. and Vulcan® XC-72 manufactured by Cabot Corp., and the conducting channel black includes Corax® L manufactured by Degussa Co. Also useful are modified furnace blacks commercially available as Ketjen® Black EC and Ketjen Black EC-600JD from Ketjen Black International. Preferred among others is acetylene black because it is more conductive due to a reduced impurity content and a well developed secondary structure. Also, Ketjen Black EC and Ketjen Black EC-600JD are useful because they have an extremely high specific surface area so that sufficient conductivity is accomplished with a low loading. The conductive carbon black may be used alone or in admixture of two or more types, in an amount of from 5 to 100 parts, more preferably from 10 to 70 parts by weight per 100 parts by weight of organopolysiloxane (A). Less than 5 parts of carbon black would sometimes fail to provide desired electric conduction whereas more than 100 parts of carbon black would sometimes result in cured products having low mechanical strength. Component (C) is an organic peroxide selected from organic peroxides of the general formulae (2) and (3) and mixtures thereof. In formula (2), X is a group of formula (4): - (CH 2 ) m - wherein m is an integer of 2 to 8 or a group of formula (5): In formulae (2) and (3), each of R 2 and R 3 which may be identical or different is a monovalent hydrocarbon group having from 3 to 10 carbon atoms such as, for example, n - propyl, isopropyl, t - butyl, n - butyl, n - amyl, t - amyl, hexyl, heptyl, octyl, 2 - ethylpropyl, 2 - ethylbutyl, 2 - ethylhexyl, and cumyl groups. Alternatively each of R 2 and R 3 is a trimethylsilyl, triethylsilyl or triphenylsilyl group of the formula: -SiR 4 / 3 wherein R 4 is a methyl, ethyl or phenyl group. In formula (3), n is an integer from 1 to 4. The organic peroxide (C) is effective as a vulcanizing agent for organopolysiloxane (A) and is blended in an amount of 0.1 to 5 parts, more preferably 0.5 to 3 parts by weight per 100 parts by weight of organopolysiloxane (A). Often less than 0.1 part of the organic peroxide would be less effective for vulcanization, resulting in cured products which are practically unacceptable because of poor properties. More than 5 parts of the organic peroxide would sometimes adversely affect due to excess decomposition residue and be less cost effective. In addition to the above-mentioned components (A) to (C), the conductive silicone rubber composition has (D) at least one triazole or triazine compound blended therein for further improving shelf stability. Component (D) is a characteristic ingredient in the present composition. The addition of component (D) to the conductive silicone rubber composition comprising components (A), (B) and (C) can prevent an increase of plasticity or pseudocrosslinking and scorching phenomenon for a long period of time. A composition comprising components (A), (B) and (C) anaerobically cures with ease. The addition of component (D) can prevent such anaerobic curing. Suitable compounds having the azo linkage include benzotriazoles and benzotriazines such as benzotriazole, benzotriazole - 5 - carboxylic acid, 1,2,3 - benzotriazin - 4(3H) - one, N - (1H) - benzotriazol - 1 - ylmethyl - formamide, 1H - benzotriazol - 1 - ylmethyl - isocyanide, with benzotriazole being most preferred. Component (D) is blended in an amount of 0.01 to 5 parts, more preferably 0.1 to 2 parts by weight per 100 parts by weight of the total of components (A) to (C). More than 5 parts by weight of component (D) would sometimes induce surface tack and exacerbate physical properties. The conductive silicone rubber composition may further contain (E) an organic silicon compound having at least one hydrogen atom directly bonded to a silicon atom (≡SiH) in a molecule for the purpose of improving the surface tack of the cured product after HAV. The organic silicon compounds used herein are often linear or cyclic and may contain a minor proportion of a branched chain structure or three-dimensional structure. Exemplary are methylhydrogenpolysiloxanes terminated with a trialkylsilyl group and having a varying degree of polymerization, organopentasiloxane represented by Si[OSi(CH 3 ) 2 H] 4 , siloxane copolymers comprised of SiO 2 and (CH 3 ) 2 HSiO 1/2 units, copolymers of methylhydrogenpolysiloxane and dialkylsiloxane, polysilylalkylenesiloxane having a SiH bond, polysilanes, and polycarbosilanes. The organic silicon compound (E) is blended in an amount of 0 to 10 parts, preferably 0.05 to 10 parts, more preferably 0.5 to 5 parts by weight per 100 parts by weight of component (A). Less than 0.05 parts of the organic silicon compound would not be effective for its purpose whereas more than 10 parts would sometimes adversely affect the heat resistance and other physical properties of cured products. If desired, additives e.g. as commonly used in conventional silicone rubber compositions, may be included in the composition. Such additives include dispersants such as low molecular weight siloxanes having a degree of polymerization of up to 100, silanol-containing silane, and alkoxy - containing silanes; heat resistance enhancers such as iron oxide, cerium oxide, and iron octylate; pigments; and reinforcing fillers in the form of silica fine powder such as fumed silica and wet silica which may or may not be surface treated to be hydrophobic, group quarts, diatomaceous earth. Also useful are saturated aliphatic hydrocarbons such as isoparaffin solvent for imparting processability and moldability to the composition; mold release agents such as fatty acid metal salts and fatty acid amides; blowing agents such as azodicarbonamide and azobisisobutyronitrile; and other well - known additives used in conventional silicone rubber compositions. Additionally the conductive silicone rubber composition may further contain at least one additive selected from the group consisting of a fatty acid of formula (6), an acid anhydride resulting from molecular condensation of the fatty acid, and an acid anhydride of formula (7). R 5 (COOH) k In formula (6), R 5 is a hydrogen atom or a saturated or unsaturated, substituted or unsubstituted hydrocarbon group having 1 to 30 carbon atoms and k is an integer of 1 to 10. In formula (7), R 6 is a saturated or unsaturated, substituted or unsubstituted, polyvalent hydrocarbon group having 1 to 30 carbon atoms and p is an integer of 1 to 3. With respect to these acids, reference is made to JP-A 25393/1993 and EP 0524046. The composition may be prepared by uniformly mixing predetermined amounts of essential components (A) to (D) and optional component (E). Although the order of addition of these components is not critical, it is preferred to prepare the composition by first mixing organopolysiloxane (A) with conductive carbon black (B), adding organic peroxide (C) and component (D) to the mixture, and finally adding component (E), if desired, while kneading or milling the mixture at respective stages in a Banbury mixer or roll mill. The thus prepared conductive silicone rubber composition may be molded by any desired technique such as compression molding, injection molding, and calendering techniques and vulcanized by an atmospheric hot air vulcanization (HAV) technique whereby the composition is converted into an elastomer. Vulcanization is generally carried out with hot air at temperatures of 100 to 500°C for 5 seconds to 10 minutes although vulcanization may be accomplished even at lower temperatures if the time is extended. Also the HAV technique may be modified by combining hot air with heating by infrared heaters or with UHF heating by taking advantage of the increased UHF absorption of the composition due to carbon black blended therein. In this way, the composition is molded and cured into a conductive silicone rubber, typically a void - free, surface tack - free, heat resistant elastomer. Therefore, the conductive silicone rubber compositions as disclosed herein are advantageously used for the manufacture of EMI shielding gaskets, building gaskets, conductive roll members such as charging rolls, transfer rolls., developing rolls, paper feed rolls, and fixing rolls, and the like. Particularly, the conductive silicone rubber composition may be continuously molded by employing HAV and therefore can be molded into a long article. EXAMPLE Examples of the present invention are given below by way of illustration and not by way of limitation. All parts are by weight and the viscosity is measured at 25°C. Examples 1-7 & Comparative Example 1 Several silicone rubber compositions were prepared by kneading 100 parts of an organo-polysiloxane comprised of 99.7 mol% of dimethylsiloxane units and 0.275 mol% of methylvinylsiloxane units, blocked with 0.025 mol% of a dimethylvinylsilyl group at either end of its molecular chain, and having a viscosity of 1 × 10 7 cs, 40 parts of acetylene black, and 1 part of cerium oxide in a pressure kneader to form a base compound, and then adding amounts of an organic peroxide and an additive to 100 parts of the base compound as shown in Table 1. These compositions were then measured for plasticity in accordance with JIS C-2123. Each of the compositions was shaped into a sheet which was covered with a polyethylene sheet and allowed to stand for 1 and 3 days at room temperature before the sheet was similarly measured for plasticity and examined for workability by an extrusion test. Separately, each of the silicone rubber compositions was extruded into a rod of 5 mm in diameter by means of an extruder having a diameter of 40 mm and passed through a HAV tower having a height of about 1 m at a temperature of 250°C, thereby accomplishing HAV within a residence time of 5 minutes. The resulting conductive silicone rubbers were examined for surface tack, hardness (JIS A scale), tensile strength and elongation in accordance with JIS K - 6301. The results are shown in Table 1. As is evident from Table 1, the composition having additive A, B or C added thereto (Examples 1-7) maintained constant plasticity, experienced no scorching and remained well workable over a long term as compared with the composition free of such a specific additive (Comparative Example).

Claims: A conductive silicone rubber composition comprising (A) 100 parts by weight of organopolysiloxane of the general formula (1): R 1 / aSiO (4-a)/2 wherein groups R 1 , which may be identical or different, are selected from substituted and unsubstituted monovalent hydrocarbon groups and a is a positive number in the range 1.90 to 2.05; (B) 5 to 100 parts by weight of conductive carbon black; (C) 0.1 to 5 parts by weight of at least one organic peroxide selected from those according to the following general formulae (2) and (3): wherein X is a group of formula (4) or (5): - (CH 2 ) m - m being an integer from 2 to 8, R 2 and R 3 are selected independently from monovalent hydrocarbon groups having from 3 to 10 carbon atoms and groups - SiR 4 / 3 wherein R 4 is methyl, ethyl or phenyl, and n is an integer of 1 to 4, and (D) 0.01 to 5 parts by weight, per 100 parts by weight of components (A), (B) and (C) combined, of at least one triazole or triazine compound. A rubber composition according to claim 1 in which component (D) is a benzotriazole or benzotriazine compound. A rubber composition according to claim 1 wherein component (D) is selected from benzotriazole, benzotriazole-5-carboxylic acid, 1,2,3-benzotriazin-4(3H) -one, N-(1H) -benzotriazole-1-ylmethyl-formamide, 1H-benzotriazol-1-ylmethyl-isocyanide, and mixtures of any of these. A rubber composition according to claim 3 wherein component (D) is benzotriazole. A rubber composition according to any one of (E) 0.05 to 10 parts by weight of organic silicon compound having at least one hydrogen atom bonded directly to silicon. A silicone rubber obtainable by curing a composition according to any one of claims 1 to 5. A method of making a shaped silicone rubber product in which a composition according to any one of claims 1 to 5 is hot air vulcanised.

Inventors: MATSUKAWA HIROSHI, TAKASHI MASAHARU, TSUCHIDA TOMIYOSHI, MATSUKAWA, HIROSHI, TAKASHI, MASAHARU, TSUCHIDA, TOMIYOSHI
Applicants: SHINETSU CHEMICAL CO, SHIN-ETSU CHEMICAL CO., LTD.
Application year: 1994
Main classification: C08K5
IPC codes: C08L83, C08K5, H01B1, C08K3